Is flu shot efficacy being oversold?

Study shows it reduces flu by 59%

Getting a flu shot doesn't provide as much protection as was previously reported, according to new analysis of more than 5,000 studies. Now it's time to be honest about the limitations of the vaccine to build trust with health care workers, says an international expert in risk communication.

"There is evidence that health care workers underestimate the value of the vaccine. Do they underestimate the value of the vaccine because the sales pitch hasn't been aggressive enough or because it's been too aggressive and dishonest?" says Peter Sandman, PhD, a risk communication consultant based in Princeton, NJ.

"Over the long haul, you can't ground public health in the ethics of a bad used car salesman. And that's what they've done. They've gotten away with it much longer than they would have if it wasn't public health," he says.

The evaluation of flu studies, published in Lancet Infectious Diseases, showed that even when the influenza vaccine is well-matched to the prevailing strain, its efficacy is only 59% for laboratory-confirmed flu in people 18 to 65 years of age.1 Just a couple of weeks before the study was published online, the Centers for Disease Control and Prevention revised its online information about flu vaccine effectiveness. Previously, the agency had said well-matched inactivated influenza vaccine was 70% to 90% effective. CDC now reports that "recent [randomized controlled trials] of inactivated influenza vaccine among adults under 65 years of age have estimated 50-70% vaccine efficacy during seasons in which the vaccines' influenza A components were well matched to circulating influenza A viruses."2 "We have said for a long time that there's a range of vaccine effectiveness. We have said for a long time that we need better vaccines," says CDC spokesman Tom Skinner, who says the CDC and vaccine manufacturers are working toward developing improved influenza vaccines. "But the vaccines that we have now are all that we have. Vaccine continues to be the single most important thing people can do to protect themselves against flu."

Efficacy varies widely based on age, vaccine type, and other factors, and in some cases is lower than 59%, says Michael Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy (CIDRAP) and lead author of the study.

"Overall, we just have a lot of work to do in the flu world," he says. "Overstating the effectiveness of this vaccine doesn't help anybody."

'Clarion call' for a better vaccine

Both Osterholm and Sandman agree that getting the flu vaccine is important. So why is the issue of flu vaccine effectiveness so important?

Currently, there's little incentive to invest in a new vaccine technology, says Osterholm, who urges public health officials to issue a "clarion call" for a better vaccine. "When you have a vaccine that's promoted by the public health community as being effective, and it's cheap, why would you spend a billion dollars to produce another one?" he says.

Meanwhile, health care workers already have some doubts about the efficacy of the vaccine. In an Internet-based survey of almost 2,000 health care workers conducted by the Centers for Disease Control and Prevention, only about half (54%) of unvaccinated health care workers agreed with the statement, "Influenza vaccination can protect me from getting influenza." Less than half (46%) of those unvaccinated employees said getting the flu vaccine was worth the time and expense.3

The rationale for vaccinating health care workers is to protect vulnerable patients. But Osterholm notes that there's little data to support commonly held beliefs about the flu vaccine. For example, there's no evidence that influenza vaccination can lead to "herd immunity," or prevention of outbreaks because most people are vaccinated, he says.

"We find there are a lot of facts that are stated in the flu world that hardly can be called facts," he says.

Even the widely cited 1997 article that found a reduction of mortality in long-term care when health care workers were vaccinated had a major caveat: Because of the low number of flu cases in the study, the authors stated, "[W]e do not have any direct evidence that the reductions in rates of patient mortality and influenza-like illness that were associated with HCW vaccination were due to prevention of influenza."4

"It really points out that there is a relative absence of good data to evaluate the effectiveness of these vaccines," Osterholm says.

A vaccine that prevents 59% of influenza is still a good bet, says Sandman. But he argues that it isn't good enough to support mandatory vaccination programs that require masking of unvaccinated employees. After all, about 40% of the vaccinated workers are also unprotected, he says.

"Anyone who claims they do science-based medicine and science-based health policy ought to be looking at the Osterholm study," he says.

Efficacy varies by age, vaccine

The flu vaccine remains the best tool to protect health care workers and their patients from influenza, agrees Osterholm. But he also highlights important differences in the protection it provides for different populations. This also has implications for health care worker vaccination.

Osterholm and colleagues analyzed 5,707 studies from 1967 to early 2011 and found only 31 that met their strict criteria. Vaccine efficacy was determined by randomized controlled trials that examined risk of acquiring influenza after vaccination, and vaccine effectiveness was determined by observational studies that measured relative flu risk after vaccination based on lab-confirmed influenza.

The analysis of flu studies showed that:

The live attenuated influenza vaccine (LAIV) is very efficacious for children but not for adults. In randomized, controlled studies, the LAIV showed an impressive 83% efficacy in children 6 months to 7 years old. However, there were not studies that met the inclusion criteria to demonstrate efficacy in older children. Osterholm notes that one study showed LAIV was 50% less efficacious in adults than the trivalent inactivated vaccine (TIV). "[The] differences could be related to the inability of the live attenuated viruses to infect some adults because of their past exposure to similar strains," the authors speculated.5

Although more research is needed, those results call into question the practice of giving the nasal vaccine to health care workers who want to avoid a shot. Today, if you're a young child you should be getting LAIV, and if you're an adult you should be getting TIV," says Osterholm.

Efficacy of the trivalent inactivated vaccine has not been established for children or adults over 65.

There were no randomized, controlled trials involving solely people over 65 or children aged 2 to 17, Osterholm and colleagues reported. About a third (35%) of observational studies showed significant effectiveness of the TIV against lab-confirmed flu. Meanwhile, the match of TIV to the prevailing strains did not show the expected influence. "Our study could not identify a close relationship between match and vaccine effectiveness," says Osterholm. "We had studies in years that there were very, very good matches and in years that there were poor matches in which the effectiveness data were identical."

Adjuvants produce only a modest increase in effectiveness.

Adjuvants were used in Canada and Europe to boost the effectiveness of the H1N1 pandemic vaccine. Four studies showed a median vaccine effectiveness of 69%, according to the analysis. Yet interestingly, older adults who had been exposed to the pandemic strain in their childhood apparently retained substantial immunity 60 or 70 years later. "There's a lot we don't know about the human immune response to the influenza infection and vaccine," says Osterholm.


1. a systematic review and meta-analysis. Lancet Infect Dis 2011 (published online Oct 25)

2. Centers for Disease Control and Prevention. Flu vaccine effectiveness: Questions and answers for health professionals. Updated on October 12, 2011.

3. Centers for Disease Control and Prevention. Influenza vaccination coverage among health-care personnel — United States, 2010–11 influenza season. MMWR 2011; 60:1073-1077.

4. Potter DJ, Stott MA, Roberts AG, et al. Influenza vaccination of health care workers in long-term-care hospitals reduces the mortality of elderly patients. J Infect Dis 1997; 175:1-6.

5. Monto AS, Ohmit SE, Petrie JG, et al. Comparative efficacy of inactivated and live attenuated influenza vaccines. N Engl J Med 2009; 361:1260-1267.